In non-destructive imaging, remote measures of different physical quantities are used at the exterior of an object in order to infer internal physical properties and their spatial distribution. In Geophysics, the object under investigation is the earth and the observations consist of measurements of the gravitational, magnetic, electric and/or elastic fields at the surface, in subsurface mines and/or in boreholes. The observations depend on the structure of the earth and are often the result of the interaction of a man-made perturbation on the medium. The main goal is to use these measurements to characterize the distribution of rocks and fluids in the interior of the earth. An approach to the challenge of imaging the interior of the earth is based on the simultaneous use of multiple geophysical fields with complementary sensitivity to a given structure. In particular, electric disturbances observed at the exterior of the earth may be caused by the passage of an elastic wave through a heterogeneous medium characterized by solid rock grains with pore spaces filled with different fluids (e.g., gas, oil, and water). The coupling between a seismic field and an electromagnetic disturbance is known as the seismoelectric effect, which has a long history in geophysics since the works of Ivanov in 1939 and Frenkel in 1944. The physics surrounding the seismoelectric effect is generally well understood, at least for the case of water-saturated rocks. The passage of an elastic wave through a heterogeneous medium causes movement of the fluids relative to the surrounding solid frame. This movement creates an electrical current due to the excess of electric charge contained in the pore water that is needed to balance the negative charge on the mineral surface. In turn, this current creates electromagnetic disturbances which can be observed with appropriate instrumentation at a distance.
The seismoelectric effect is sensitive to the presence of fluids in rock pores; thus, the seismoelectric effect provides a rare opportunity to characterize fluid saturation, rock permeability, and other phenomena that cannot be observed just with elastic waves. On the other hand, two main difficulties complicate the use of the seismoelectric effect in geophysical exploration and characterization. First, the seismoelectric effect is small, which limits its expression at a distance from the occurrence of a conversion of the mechanical-to-electrical signals. In general, it is estimated that the electric field disturbances can be observed up to a kilometer from the position of such a conversion. Second, seismoelectric conversions occur at any location where an elastic wave is present and at places where the medium solids and fluids are in direct contact. This limits the ability to associate an observed change in electric potential with a specific location in the subsurface.
Electrical resistivity tomography is generally used to identify and image electrical conductivity variations in the earth. These conductivity variations are then used to characterize the various properties of an area being observed. The classical way to do electrical resistivity tomography involves injecting an electrical current into the ground and measuring the resulting electrical potential received at a set of receiving electrodes. For example, an electrical current may be injected into the ground via electrodes in one borehole while the measured changes in electrical potential are observed in another, distant borehole. In other instances, an electrical current may be injected into the ground via surface electrodes and the change in electrical potential may be observed via distant surface receivers and/or borehole receivers.
One main drawback of this methodology is that the sensitivity is mainly localized to the vicinity of the electrodes, thus limiting the ability to image a wide area with high resolution. That is, to adjust the size or resolution of the area being imaged, the number and/or position of the electrodes would need to be adjusted. Adjusting the number and position of the electrodes may be cost and/or time prohibitive, especially when multiple boreholes may need to be obtained. Moreover, such additional boreholes may disturb the site being monitored both above and below surface.